Synchronous continuous casting processes are processes that do not have a relative speed difference between a casting and the inner walls of a mold. For example, such as a twin-drum process (a twin-roll process), a twin-belt process, a single-roll process and the like. A twin-drum type synchronous continuous casting process is a continuous casting process that consists of the steps of: (i) pouring molten steel into a continuous casting mold composed of a pair of cooling drums, which may have identical diameters or different diameters and may be disposed in parallel or with an inclination relative to each other, and side weirs for sealing both end faces of the cooling drums; (ii) forming a solidified shell on the circumference of each of the cooling drums; and (iii) uniting the solidified shells near a position where the rotating cooling drums come closest to each other (the so-called “kissing point) to form a united thin strip casting.
It is known that surface defects (e.g., unevenly glossy defects on the surface of a cold-rolled product and rough surface defects on the surface of a formed product) are sometimes generated along the rolling direction of a product. For example, surface defects may be formed when the product is produced by cold rolling, with hot rolling not applied beforehand, and thin strip casting through a twin-drum type continuous casting process or the like, when cold forming (e.g., draw forming or stretch forming) is applied thereto. These surface defects are generated in a different manner from the “orange peel phenomenon,” which depends on the diameter of the crystal grains of a cold-rolled product, individually or compositely. In particular, the defects may be in the forms of: (1) small undulated surface defects not more than several millimeters in length and not more than 0.5 mm in width on average; and (2) large stream patterned surface defects not more than several hundred millimeters in length and not more than 3 mm in width on average. For example, these surface defects may be observed when a BA product (a product produced through bright annealing) is subjected to stretch forming and may deteriorate the appearance of the formed product.
The small undulated surface defects, of not more than several millimeters in length and not more than 0.5 mm in width, may be generated in steel where δ-ferrite remains in an austenite phase. These surface defects may be caused by the uneven structures formed on the surfaces of a casting as a result of the variation of the residual amount of δ-ferrite due to the heat history of the casting. Thus, the positions where the surface defects are generated on the top and bottom surfaces of a steel sheet are not identical with each other. Japanese Patent Publication No. H5-23861, the entire disclosure of which is incorporated herein by reference, discloses a method of preventing surface defects on a steel sheet product by adjusting the interval of dimples on the surfaces of the cooling drums. Additionally, Japanese Patent Publication No. H5-293601, the entire disclosure of which is incorporated herein by reference, discloses a method of eliminating δ-ferrite on the surface layers of a casting by delaying the cooling of the casting after it comes out of a high temperature mold. Further, Japanese Patent Publication No. 2000-219919 the entire disclosure of which is incorporated herein by reference, discloses a method comprising the steps of: (i) casting a thin strip casting; (ii) imposing a strain to the vicinity of the surfaces of the casting through shot blasting; and (iii) annealing. Thus, recrystallization of the strained surface during annealing creates uniformly sized crystal grains and therefore removes the surface gloss.
The large stream patterned surface defects, not more than several hundred millimeters in length and not more than 3 mm in width, are caused by the local variation of deformation resistance due to uneven distribution of Ni segregation (e.g., normal segregation and inverse segregation) remaining at the finally solidified portion of a casting, e.g., at a portion in the middle of the thickness of a steel sheet product. These surface defects are generated at identical positions on both the top and bottom surfaces of a steel sheet. Japanese Patent Publication No. H7-268556, the entire disclosure of which is incorporated herein by reference, discloses that Ni segregation is mitigated by performing casting while the degree of superheat ΔT of molten steel is controlled to not higher than 50° C. during continuous casting and thus minimizing the flow of the molten steel at the finally solidified portion.
Japanese Patent No. 2851252, the entire disclosure of which is incorporated herein by reference, discloses that Ni segregation is caused by semisolidified molten steel, which is in a state close to final solidification and has a solid phase ratio of less than about 1.0, is moved in the direction of the sheet width or in the direction of casting by a driving force. This driving force is created by the pressing force P of a mold, imposed when a casting is formed by sticking the solidified shells together on the mold wall faces. Consequently, Ni segregation may be mitigated and therefore reduce surface defects by defining the pressing force P as a function of a degree of superheat ΔT of molten steel and controlling the pressing force P to roughly not more than 5 t/m, and more particularly to controlling the pressing force P to about 2.5 t/m.
By the various corrective measures described above, the surface defects generated when a product produced by cold-rolling a thin strip casting is subjected to cold forming have been significantly improved. However, it has been found that previously unknown minute surface defects may be generated. These new surface defects are sometimes recognized as unevenly glossy defects at the stage of a cold-rolled steel sheet in the same way as before, but are far finer and smaller than the previously known defects. Further, when these new defects are very small they are not recognized as unevenly glossy defects at the stage of a cold-rolled steel sheet or after usual cold forming but are found as minute rough surface defects after excessive cold forming is applied, e.g., deep drawing or stretch forming, which may cause problems in some applications. Therefore, these defects must be eliminated in cold-rolled steel sheet applications, e.g., where buffing after forming is omitted.
As described above, the conventional large stream patterned surface defects are generated at identical positions on both the top and bottom surfaces of a steel sheet. The protrusions and depressions thereof are distributed in the form of streaks or lines with a height difference between a protrusion and a depression of about 1 to 3 μm. A Ni segregation portion is located where the conventional large stream patterned surface defect is generated, with normal segregation and inverse segregation existing in the form of bands in the middle of the sheet thickness. In contrast, although the newly found surface defects are generated at identical positions on both the top and bottom surfaces of a steel sheet the protrusions and depressions are distributed sporadically and in a zigzag pattern in the form of spots, with a length of several tens of millimeters and a height difference between protrusions and depressions of from about 0.1 μm to about 1 μm. Thus, these newly found surface defects have been named “salt-and-pepper unevenly glossy defects” at the stage of a cold-rolled steel sheet. At a portion where a salt-and-pepper unevenly glossy defect is generated in the middle of the sheet thickness, an Ni inverse segregation portion exists and normal segregation does not exist in the adjacent vicinity. In this respect, a salt-and-pepper unevenly glossy defect is differentiated from a conventional rough surface defect where both normal segregation and inverse segregation coexist.